The present disclosure relates to an apparatus and method of measuring data in a high voltage direct current (HVDC) system, and more particularly, to an apparatus and method of measuring data in a high voltage direct current (HVDC) system that marks time information with sensor or meter data in order to reduce an error that occurs because a data acquisition time at each sensor or meter is different from each other, to maintain the accuracy of control.
In recent, a demand for high voltage direct current (HVDC) is soaring due to an increase in a large offshore wind farm and smart grid establishment.
The HVDC is a technique that enables a high-voltage alternating current (AC) generated by a power station to be converted and transmitted into a DC by using a power converter and then re-converted into an AC at a power receiving point for power supply.
Such an HVDC transmission technique may stably transmit power over long distances because there is little power loss, insulation is easy due to a relatively low voltage compared to an AC, and there is little induced obstacle, thus the transmission technique enables efficient, economical power transmission and may overcome the drawbacks of AC power transmission.
FIG. 1 schematically shows an HVDC transmission technique.
Referring to FIG. 1, an HVDC system 100 is installed between an AC network A (hereinafter, referred to as “system A”) (110) and an AC network B (hereinafter, referred to as “system B”) (120) to link the two systems 110 and 120.
The HVDC system 100 converts and transmits AC power received from any one 110 or 120 of the two systems 110 and 120 into DC power, and a power receiving point that has received the DC power re-converts the DC power into the AC power to transmit the AC power to the other system 110 or 120.
In particular, the HVDC system 100 may convert and transmit the AC power received from the system A 110 into DC power and a power receiving point that has received the DC power may re-convert the DC power into the AC power to transmit the AC power to the system B 120, or the HVDC system may convert and transmit the AC power received from the system B 120 into DC power and a power receiving point that has received the DC power may re-convert the DC power into the AC power to transmit the AC power to the system A 110.
Each of the system A 110 and the system B 120 may transmit AC power to the HVDC system 100 or receive AC power from the HVDC system 100.
In this example, the system A 110 and the system B 120 may be AC power systems in the same country or be AC power systems that use the same frequency. However, they may also be AC power systems that are used in different countries or use different frequencies, according to an embodiment. In this case, the HVDC system 100 enables a link between countries or between AC power systems that use different frequencies.
FIG. 2 shows the flow of measurement data in a typical HVDC system.
An AC and DC measurement device 210 measures AC data and DC data. To this end, the AC and DC measurement device 210 may include at least one current transformer (CT) and potential transformer (PT) that measure AC data and DC data.
The AC CT senses the current of an AC bus at which the AC CT is installed, to measure the AC current. The AC PT measures an AC voltage. Also, a DC CT, a DC voltage divider (VD) for measuring the current and voltage of a DC are installed to measure analog state values. In this case, the DC CT measures the DC current and the DC VD measures the DC voltage.
A substation automation system (SAS) 220 obtains operation information on a facility rapidly and accurately. The SAS 220 classifies the above-described measurement data into AC and DC related information, analog information, and digital information to transmit the measurement data to an upper control system 240 through a remote terminal unit (RTU) to be described below.
The RTU 230 receives data classified by the SAS 220 and transmits it to the upper control system 240.
The upper control system 240 may be a supervisory control and data acquisition (SCADA) or an energy management system (EMS). The upper control system 240 manages and controls a system through state estimation, power system power flow analysis and credible accident simulation based on the received data.
In order to perform control needed for power system operation, an input data based operation is needed. In this case, although input data used for an operation is used on the assumption that it has been simultaneously measured, it is actually difficult to secure the synchronism of all measurement data.
It is because time synchronization is not made by many kinds of differences that occur in the process that data measured by using a passive sensing device is transmitted to an operation unit. In particular, the differences include a difference in transmission speed due to a difference in distance between a sensing device and a control unit, a time delay according to the device characteristic of each sensor, and a difference in time that is consumed to convert analog data into digital data that may be used for a control operation.
FIG. 3 is a diagram for explaining that the measurement time of data in a typical HVDC system has asynchronism.
It is assumed that the HVDC system performs a control operation based on four types of analog information obtained in FIG. 3, i.e., an AC current 310, a DC current 320, an AC voltage 330, and a DC voltage 340.
A control operation time is a time when an operation for control is performed. In FIG. 3, the HVDC system performs an operation at time A indicated by a dotted line. In the case where the operation is performed at the time A, the HVDC system would perform the control operation based on the four types of analog information obtained at the time A.
In this case, the measurement time of each of the four types of analog information used for the operation is shown by an arrow. Referring to FIG. 3, times corresponding to arrows in the AC current 310, the DC current 320, the AC voltage 330, and the DC voltage 340 are different from each other.
That is, although the HVDC system performs a control operation based on the four types of analog information obtained at corresponding times at the control operation time A, the respective measurement times of the analog information that the HVDC system has obtained at the time A are different from each other. That is, the respective measurement times of the four types of analog information are not synchronized.
As described earlier, it results from the difference in transmission speed due to the difference in distance between the sensing device and the control unit, the time delay according to the device characteristic of each sensor, and the difference in time that is consumed to convert the analog data into the digital data that may be used for the control operation.
Thus, in the case where the operation is performed by using pieces of data having asynchronous measurement times, the final control operation result may have an error because the respective measurement times of data are different from each other and due to the resulting difference and error in the AC current, the DC current, the AC voltage.
As such, pieces of data obtained at a time when the system performs the operation have a difference in measurement time between pieces of obtained data, and thus in the case where the operation is performed, an error occurs and thus the accuracy of control decreases, and further, control for second and third correction should be accompanied. In this case, a control target value does not converge and continues to vibrate and in the worst situation, the value may diverge.